Upgrade the GCS SDK to v1.2.1

Since v0.46.2 of the GCP SDK, the storage client has been released independently and made available as a submodule, tagged with storage/vX.Y.Z.

We're currently using the GCP SDK v0.47.0. The version of the storage client bundled with this version of the SDK is v1.1.1.

This MR upgrades the storage client to v1.2.1. I've tried to upgrade it to the latest version but failed (see #39 (closed)).